Efficient and promising oxidative desulfurization of fuel using Fenton like deep eutectic solvent

Oxidative desulfurization (ODS) has emerged as a prominent technique for the removal of sulfur compounds from fuels, aiming to comply with stringent environmental regulations and minimize sulfur dioxide emissions. Herein, Fenton-like deep eutectic solvents (DESs) were synthesized as a catalyst and reaction medium and their application for the ODS process was investigated. The study encompassed the optimization of DES composition, reaction conditions, and the influence of different parameters on the desulfurization efficiency. The experimental findings demonstrated that the Fenton-like DES exhibited outstanding catalytic activity in the oxidative desulfurization of fuel. The optimized conditions involved conducting the reaction at room temperature for 2.5 h, using 200 mg of the prepared DES (HNFM-FeCl4) as both the extraction solvent and catalyst. An oxidant-to-sulfur (O/S) ratio of approximately 3:1 was maintained, with a 30 wt% H2O2 solution utilized as the oxidant. The analysis of the reaction products using GC–MS revealed a remarkable yield of 98% for dibenzothiophene sulfone. The DES provided a suitable medium for the reaction, enhancing the solubility and availability of sulfur compounds. The iron catalyst, in the presence of hydrogen peroxide, facilitated the oxidation of sulfur-containing compounds to their corresponding sulfones, which can be easily separated from the fuel phase. The DES catalysts exhibited stability and recyclability, making them suitable for practical applications in fuel desulfurization processes.

www.nature.com/scientificreports/These properties include a low melting point, high thermal stability, non-flammability, and negligible vapor pressure [22][23][24][25] .DES also has the advantage of being customizable, low cost, and simple preparation, as their physical and chemical properties can be tailored by selecting specific components [26][27][28] .The use of DES in ODS offers several benefits [29][30][31][32] .They provide a suitable medium for the reaction, enhancing the solubility and availability of sulfur compounds [33][34][35][36] .DES can selectively extract sulfur compounds from the fuel matrix, facilitating their subsequent oxidation 37 .Furthermore, DES exhibit good stability and can be easily recycled, making them environmentally friendly and economically viable for practical applications [38][39][40] .Researchers continue to explore and optimize the use of DES in ODS processes, seeking to improve desulfurization efficiency, reduce costs, and address any challenges that may arise [41][42][43][44] .The development of DES-based systems for ODS holds promise for the production of cleaner fuels with significantly reduced sulfur content, contributing to environmental sustainability and compliance with stringent regulations.
In our research, our main focus was to develop a facile and straightforward deep eutectic solvent (DES) that could act as a catalyst and reaction medium in organic transformations [45][46][47] .Herein, we reported on the synthesis of a Fenton-like DES based on N-formyl morpholine and iron salts as a green solvent system that could serve as both a catalyst and a reaction medium for oxidative desulfurization while maintaining environmentally friendly conditions.

Experimental General
The chemical compounds and solvents used in our research, including N-formyl morpholine (NFM), DBT, BT, MPS,FeCl 3 ⋅6H 2 O, HCl 37%, commercial salts were purchased from supplier.To characterize the DES catalyst, various methods were analytical instrument were employed.FT-IR spectra were acquired using a Bruker Vector-22 infrared spectrometer with KBr disks, enabling the identification and characterization of chemical bonds and functional groups present in the samples.For elemental analysis, EDX (Energy-dispersive X-ray spectroscopy) studies were conducted using a scanning electron microscope (VEGA3 TESCAN) operating at 20 kV, providing quantitative information about the elemental composition of the samples.To determine the melting point of the compounds, a Büchi 535 melting point apparatus was utilized.This allowed for the measurement of the temperature at which the substances transitioned from solid to liquid phase, providing valuable information about their thermal properties.Thermal gravimetric analysis (TGA) was performed using a Netzsch-TGA 209 F1 instrument under a nitrogen atmosphere.The TGA data was obtained by subjecting the samples to a heating rate of 25 °C/min, ranging from room temperature to 850 °C.This technique enabled the study of the thermal stability and decomposition behavior of sample as a function of temperature.In addition, the sulfur content was determined by gas chromatography (Agilent 7890A) coupled with a flame ionization detector (GC-FID).Separation was achieved using an HP-5 capillary column with dimensions of 30m × 0.32mm inner diameter × 0.25 μm film thickness.

Preparation of DES
The preparation of the DES involved two steps, as outlined below.
In a round bottom flask, 100 mmol of N-formylmorpholine (NFM) and 100 mmol of HCl 37% were mixed and was heated under stirring at 50 °C for 24 h.The resulting mixture was then subjected to evaporation using a rotary evaporator until a white solid, HNFM-Cl, was formed.In the following steps, to the HNFM-Cl obtained from Step 1, 100 mmol of commercial salts (Table 1) for example FeCl 3 was added.The mixture was further

Result and discussion
After synthesizing the desired DES using a straightforward method, it is crucial to characterize it using various spectroscopy and thermal analysis techniques.In the first, the DES was analysed using FTIR spectroscopy (Fig. 3), which revealed several significant absorption bands.One notable absorption occurs at approximately above 2935 cm −1 , indicating the presence of a tertiary ammonium salt within the DES.Another absorption peak around 2935 cm −1 corresponds to C-H aliphatic stretching vibrations.The presence of an amidic carbonyl group is evident from a strong and sharp absorption band at approximately 1643 cm −1 , which can be attributed to C=O stretching.The absorption band at around 1455 cm −1 is associated with methylene C-H bending vibrations.Additionally, a pronounced absorption band at 1104 cm −1 is observed, which can be attributed to C-O stretching.
The thermal properties of the deep eutectic solvent (DES) were investigated using the thermogravimetric analysis (TGA).The TGA results revealed three major mass losses, along with some minor losses, which confirmed the structural composition of the DES (as shown in Table 1 and Fig. 4).The minor mass loss observed at   21 .The residue char remaining at 800 °C was found to be 7.5%, confirming the presence of an organic-inorganic hybrid structure in the DES.Additionally, the stability of the DES (HNFM-FeCl4) was evaluated by subjecting it to increasing temperatures.The results, as shown in Table 1, indicated that only a 5% mass loss occurred until 223 °C, and a 10% mass loss was observed at 243 °C.After reaching temperatures higher than 243 °C, the DES (HNFM-FeCl4) started to decompose.These findings demonstrate the remarkable stability of the DES up to these temperatures, suggesting its suitability for applications that require thermal resilience.The surface morphology of the DES was investigated using SEM technique.The SEM analysis revealed a distinct and uniform surface vicissitude on the DES, indicating potential suitable sites for the progression of oxidation reactions.In addition to the SEM analysis, energy-dispersive X-ray spectroscopy (EDS) was performed to obtain elemental composition information (Table 2 and Fig. 5).The EDS image showed a homogeneous distribution of elements throughout the DES.The EDS statistical results confirmed that the weight percent of iron (Fe) in the DES matched the reported char yield and the expected mass present in the molecular formula.Furthermore, mapping images obtained from the EDS analysis demonstrated the homogeneity of the DES.These findings are presented in Table 2 and Fig. 5, providing visual evidence of the uniform distribution of elements within the DES.The combined SEM and EDS analysis contributes to a comprehensive understanding of the surface morphology, elemental composition, and homogeneity of the DES.
Following the successful preparation and characterization of the DES, the optimization of the oxidation reaction was undertaken by exploring various reaction conditions.Several parameters were investigated to improve the efficiency of the process.These parameters included the selection of anion salt, temperature, catalyst quantity, oxidant type, oxidant-to-sulfur ratio, and reaction duration.By systematically studying these factors, the aim     and Fe(NO 3 ) 3 were used to prepare NMF based DES and results were shown in Table 3.Each DES was evaluated for its efficiency in oxidizing DBT in the organic substrate.The results obtained are summarized in Table 3, with the oxidation efficiencies of DBT being determined as follows: NiCl 2 (56%), SnCl 2 (43%), ZnCl 2 (54%), CoCl 2 (48%), CrCl 3 (51%), SbCl 3 (46%), MnCl 2 (39%), Fe(NO 3 ) 3 (35%), FeCl 2 (71%) and FeCl 3 (85%).Based on these results, FeCl 3 was identified as the most effective salt for oxidizing DBT, exhibiting a high oxidation efficiency of 90%.Therefore, FeCl 3 was selected as the optimal salt for further experiments and process optimization.
In the second step of optimization, the mass of the DES was investigated to determine its effect on the oxidation of DBT.Initially, without the DES, the yield of oxidation was only 7%, indicating the limited effectiveness of the reaction in the absence of the DES.To optimize the DES mass, different amounts of DES were loaded into the reaction system.Loading 50 mg of DES resulted in a significantly increased yield of oxidation (72%).Further increasing the DES mass to 100, 150, 200 250 and 300 mg led to yield of 85%, 95%, 98%, 98% and 98% respectively.However, upon further increasing the amount of DES, no significant improvement in the yield of oxidation was observed.These results, as summarized in Fig. 6, demonstrate the optimum DES mass was found to be 200 mg, which achieved a high yield of 98% oxidation.Beyond this point, increasing the DES mass did not provide any additional benefits in terms of improving the oxidation yield and times.
In the optimization process, another important step involved optimizing the molar ratio of hydrogen peroxide (H 2 O 2 ) to DBT and results were shown in Fig. 7.While stoichiometric ally, 2 mol of H 2 O 2 are required to fully oxidize DBT to dibenzothiophene sulfone (DBTO 2 ), it is important to consider that reactions may not proceed with 100% efficiency.Therefore, a higher molar ratio of reactants is often needed to ensure complete conversion.To determine the optimal H 2 O 2 molar ratio to DBT, seven different ratios of n(H 2 O 2 )/n(S) including 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, and 4:1 were tested: The corresponding sulfur removal yields were found to be 35%, 42%, 65%, 81%, 95%, 98%, 98% and 98%, respectively.Interestingly, the results indicate a direct proportionality between the molar ratio of H 2 O 2 to DBT (O/S ratio) and the sulfur removal yield.As the O/S ratio increased from 0.5:1 to 3:1, the sulfur removal yield steadily improved.However, beyond an O/S ratio of 3:1, there was no significant increase in the sulfur removal yield.These results suggest that an O/S ratio of around 3:1 is optimal for achieving high sulfur removal yields in the DBT oxidation reaction.
In the final step of optimization, the time and temperature conditions for the oxidation reaction were investigated (Fig. 8).Three different temperatures, 30 °C, 40 °C, and 50 °C, were tested to identify the most effective temperature for the reaction.The results indicated that the most effective temperature for developing the oxidation reaction was reported to be 30 °C, followed by 40 °C and 50 °C, respectively.It is important to note that at higher temperatures, there is a higher likelihood of hydrogen peroxide decomposition, which can contribute to a reduction in sulfur removal efficiency.In addition to temperature, the reaction time was also considered.It was observed that extending the reaction time beyond 2.5 h did not significantly increase the effectiveness of the oxidation process.Therefore, it can be inferred that the reaction reaches its maximum efficiency within this time frame.These findings suggest that the optimal temperature for the oxidation reaction is 30 °C, and it is recommended to keep the reaction time within a range of approximately 2.5 h.
By subjecting the reaction mixture to GC-MS analysis, it is possible to assess its purity, detect any impurities or byproducts, and gain insights into its chemical composition.According to the GC-MS analysis conducted under optimized conditions, (Fig. 9) it was found that DBT was converted to dibenzothiophene sulfone with a yield of 98.51%.
In order to assess the reusability of the DBT extraction solvent and catalyst (DES) under optimized conditions, a scaled-up reaction was performed using 200 mg of DES.Once the reaction was completed, to separate the DES, ethyl acetate (10 mL) was added.The mixture was then subjected to centrifugation, allowing for the separation of the DES, which was subsequently washed with ethyl acetate (5 mL).After the washing step, the DES was dried.This process was repeated for four consecutive runs, and the results are presented in Fig. 10.The data obtained from the experiments demonstrated that the DES could be recycled and reused for at least five cycles without experiencing a notable decrease in yield, as depicted in Fig. 10.
The fact that the reused DES did not showed any discernible differences compared to the fresh DES in the FTIR analysis, as depicted in Fig. 11, suggests that the DES remains stable in the desulfurization system.This stability is an encouraging finding as it indicates that the DES can be reused without significant degradation or alteration in its chemical composition and properties.
Based on the literature [48][49][50] and information in this article, a plausible mechanism has been proposed for this system (Fig. 12).Although the Fenton-like reagent (Fe 3+ /H 2 O 2 ) has been widely used in organic oxidations, the exact mechanism of its action is still not fully understood.Considering that the DES solvent is immiscible in the model oil, the oxidation process is therefore a two-step process.In the first step, DBT is extracted in the DES phase, and in the next step, sulfur is oxidized in the sulfone.The DES acts similarly to the Fenton-like reagent and produces hydroxide radicals, which in the next step, cause the oxidation of sulfur to sulfoxide.

Conclusion
In summary, this study investigated the application of Fenton-like DES as catalysts and reaction media for the oxidative desulfurization of organosulfur compounds contained in fuels at ambient temperature and atmospheric pressure.The various low cost metal salts based DES were designed and used as catalyst and extractant   www.nature.com/scientificreports/for ODS process.Among the DESs studied, the iron-based DES, in combination with hydrogen peroxide as the oxidant, exhibited the highest catalytic activity for the oxidation of sulfur-containing compounds to sulfones.
The DES system demonstrated stability and recyclability, rendering it suitable for practical implementation in fuel desulfurization processes.The ability to recover and reuse the DES enhances its economic viability and sustainability.

Figure 1 .
Figure 1.Left: a polluted day in west of Tehran on January 12th 2021, right: a clear day in west of Tehran on January 16th 2021.

Table 3 .
The effect of various metal salts based DES.

Figure 8 .
Figure 8. Temperature and Time optimization.

Figure 9 .
Figure 9. GC-MS analysis of reaction mixture under optimized conditions.

Table 1 .
Thermal properties data of DES in N 2 atmosphere.

Table 2 .
EDS data of DES.

Table 4 .
The sulfur removal for different sulfur compounds.